1. Field of the Invention
The present invention relates to an image processing apparatus used in a copying machine or a facsimile system and, more particularly, to an image processing apparatus having a color ghost correction function and capable of processing with high quality a black character or the like included in a color image.
2. Description of the Prior Art
A conventional image processing system comprises an image reading unit for reading a document image and converting the document image into red (R), green (G), and blue (B) image signals, a means for color-correcting (converting) these R, G, and B signals into Y (yellow), M (magenta), and C (cyan) signals, an undercolor removing means for extracting achromatic components from the Y, M, and C signals and generating a K (black) signal, a multi-value converting means for converting the Y, M, C, and K signals into multi-value signals, and an image recording unit for producing a hard copy of the document in accordance with the multi-value Y, M, C, and K signals.
The document image is converted into the R, G, and B signals by the image reading unit. The R, G, and B signals are color-corrected into the Y, M, and C signals representing complementary colors of the colors represented by the R, G, B signals. An image is finally recorded by the Y, M, and C toners. When all colors are expressed by the Y, M, C colors, a lack of density in a high-density portion (black or the like) of an achromatic color tends to occur. For this reason, equivalent achromatic components are extracted from the Y, M, and C signals by the undercolor removing means, and the black (K) signal is generated (undercolor removal or UCR). In order to provide sufficient gradation levels to the image recording apparatus, the Y, M, C, and K signals are converted into multi-value (2 to 8 values) signals. In a normal operation, an image requires 32 or more gray scale levels. For this reason, a dither method or the like is combined with an area gradation method to perform multi-value conversion for gradation expressions. The multi-value Y, M, C, and K signals are output as an image from the image recording apparatus.
When a reading unit for reading a document by photoelectric transducer elements and simultaneously producing B, G, and R signals is used as an image data input means, positional errors of light incident on the photoelectric transducer elements occur by color separation beams due to various causes in an optical system. As a result, a color ghost is formed in a recorded image. In the color ghost phenomenon, edge portions of a black character or figure are output as chromatic portions, and image quality is therefore degraded. It is possible to predict appearance of a color ghost to some extent from dot patterns and the like of the output image. In this case, however, it must be determined whether a dot of the output image is black. In addition, dot patterns must be stored in a memory. In order to realize this, a circuit arrangement becomes bulky and complicated.